The present invention relates to a golf ball mold suitable for molding solid golf balls composed of a core encased by one or more cover layer, and thread-wound golf balls. The invention relates also to golf balls molded using such a mold, and to a method of manufacturing golf balls using such a mold.
Molds for molding golf balls are generally composed of a plurality of parts which removably mate to each other; a golf ball is manufactured by feeding a golf ball molding material to a cavity that forms at the interior of the mold when these mold parts are mated. From the standpoint of ease of mold fabrication and ball moldability, etc., the parting plane of the mold parts is often rectilinear in shape without concavities and convexities. A parting plane having such a rectilinear shape is often coincident with the equator of the golf ball. Thus, in golf balls molded with such a mold, dimples are not formed on the equator which corresponds to the parting plane; instead, a somewhat wide great circle is formed at the equator.
However, in a golf ball having at the equator a wide great circle with no dimples lying thereacross, it is difficult to achieve a uniform arrangement of dimples on the spherical surface of the ball. This leads to a lack of uniformity in the aerodynamic symmetry of the ball, giving rise to a variability in the flight performance depending on where the ball is hit.
Hence, to eliminate a wide great circle on the equator, attempts are being made to form dimples that lie across the equator. For example, JP-A 10-127826 discloses a golf ball mold 10 having a construction wherein, as shown in FIG. 8, an upper mold half 10a and a lower mold half 10b removably mate to form at the interior a hollow spherical cavity c having an inner wall with numerous dimple-forming protrusions 40 thereon. In addition, the parting planes 30 on the upper and lower mold halves are formed in concavo-convex shapes, and dimple-forming protrusions 40 are situated so as to lie across the parting line PL at the concavo-convexly shaped areas. In this mold 10, to form dimples which lie across the parting line PL, cylindrical pins (convex features) 30a having dimple-shaped ends are provided on the parting plane 30 of the lower mold half 10b and circular holes (concave features) 30b corresponding to the cylindrical pins (convex features) 30a are formed on the parting plane 30 of the upper mold half 10a so that these fit together when the upper and lower mold halves are mated. Also shown in the diagram is a resin injection port 20.
In addition to the foregoing, numerous disclosures have been made wherein, to have dimples lie across the golf ball equator, the parting plane of the mold is given a shape that is concavo-convex rather than rectilinear, with portions of, or entire, dimple-forming protrusions being disposed on the convex portions thereof (e.g., JP-A 06-143349, JP-A 08-173576, JP-A 11-070186, JP-A 11-137727, JP-A 2001-170217, JP-A 2001-187172, JP-A 2002-159598, JP-A 2004-089549, JP-A 2006-212057, JP-A 2007-136182, JP-A 2007-159715 and JP-A 2007-268265).
At the same time, in terms of the aerodynamic properties of a golf ball, it is desirable to provide dimples on a greater portion of the golf ball surface, such advantageous effects being known to increase as the total surface area of the dimples as a proportion of the golf ball surface area (surface area coverage) approaches 100%. Hence, to improve the symmetry of the aerodynamic properties in this way and achieve an even further increase in performance, bringing the dimple surface coverage close to 100% is important.
In this type of mold, resin injection ports (also referred to below simply as “injection ports”) for introducing resin into the cavity are normally provided along the parting plane. However, circular injection ports having a specific opening surface area in order to keep imbalances in the injection pressure and resin flow rate from arising are generally formed at sizes and positions which do not overlap with the dimple-forming protrusions (on the molded ball surface, at positions which correspond to lands where dimples are not formed). For example, JP-A 2000-42143 mentions providing circular injection ports having a diameter of about 0.5 to about 1.0 mm. However, the intervals between the dimples become smaller as the dimples are arranged more densely so as to bring the dimple surface coverage closer to 100%; as a result, the surface area available for providing injection ports decreases. This problem is commonly addressed by reducing the diameter of the injection ports, but imbalances in the resin injection pressure and flow rate often arise as a result. Hence, when a cover is formed over a core or a sphere composed of a core encased by another layer such as an intermediate layer, the core may end up deformed or the position of the core within the mold may end up off-center, leading to various problems, such as molding defects, scorching or eccentricity. Golf balls with these problems have an inferior quality, such as durability, symmetry and appearance. Also, as shown in FIG. 9, by adjusting the sizes of the dimple-forming protrusions 40 and making the interval between those dimple-forming protrusions 40 which adjoin the injection port 20 larger, it is possible to secure the surface area of the opening in the injection port 20, although further increase in the uniformity of the aerodynamic properties or the surface coverage is difficult.
Thus, various innovations have hitherto been made to increase the dimple surface coverage and thereby enhance the aerodynamic properties of the ball. However, a fundamental solution to the conflicting problems of mold design and dimple design has yet to be achieved. Accordingly, an approach that resolves the above problems has been sought, both in order to further enhance the aerodynamic properties of the golf ball and also to improve the degree of freedom in mold design.
Also, in the prior art, because the basic shape of an injection port is circular, it is difficult to efficiently cool the entire gate area for which the injection port serves as the opening; excessive time has thus been required to cool gate areas. Molding defects sometimes arise at the ball surface near the gates, particularly in cases where a resin material of relatively low hardness is molded. That is, after the resin material has been injection-molded, even when an effort is made to cool the entire gate area, cooling of the resin material within the gate near the center axis thereof is inadequate. As a result, after the resin material within the gate has been cooled and solidified, a gate mark ends up remaining on the surface of the finished ball obtained by parting the top and bottom mold halves, removing the molded ball, and subjecting it to gate cutting treatment, trimming and painting. Such gate marks are conspicuous and detract from the appearance of the ball.
Examples of prior art directed at injection gates having an injection port in a golf ball mold include JP-A 2000-185117, JP-A 2000-185116, JP-A 09-313647 and JP-A 08-034036.